1. Field of the Invention
The present invention relates to adhesive products such as label stock, labels, tapes and the like, and more particularly to adhesive products having protective backings provided with embrittled zones that aid in removing the backing from the adhesive.
2. Description of the Prior Art
Many adhesive products, particularly those having pressure sensitive adhesives, are provided with a backing which is secured to the surface of the adhesive and which protects the adhesive before use. It is known to provide embrittled zones in the form of thin lines running across the backing of such adhesive products. When the adhesive product is sharply flexed along an embrittled line, the backing cracks along the line and the newly-formed edges of the backing lift up off the adhesive layer, thereby offering gripping points for peeling the backing from the adhesive. Embrittled zones that are not sharply flexed have sufficient tensile strength to permit removal in one continuous piece of a portion of backing having one or more of such embrittled zones. Thus, when two or more embrittled zones are present in the backing of a label, one of the zones can be cracked selectively and the backing can be removed in only two pieces. Until the flexing operation occurs, however, the adhesive surface is protected and contained by the backing. The embrittled zones are integral with the backing and serve to protect and contain the adhesive until cracked.
U.S. Pat. No. 3,035,957, incorporated herein by this reference, describes one such adhesive product having a fibrous backing with embrittled zones. The embrittled zones are produced by applying heat, a flame, a chemical substance such as sulfuric acid, ultraviolet radiation, or the like to partially degrade or decompose a portion of the fibrous backing. U.S. Pat. No. 3,006,793, incorporated herein by this reference, describes adhesive products in which embrittled zones extend diagonally across the backing from edge-to-edge. In one embodiment, the embrittled zones are produced by applying a chemical embrittling agent such as an aqueous solution of sulfuric acid with the use of a printing roll having raised helical printing ridges.
Japanese Patent Application No. 1971-11392, which was filed on Mar. 5, 1971, and laid open to public inspection on Nov. 14, 1972, and is incorporated herein by this reference, describes forming the embrittled zones by applying lines of viscous, aqueous solution of hydrochloric acid with the use of an applicator roller having helical grooves which receive the acid solution from a dip bath and transfer the solution to the backing. The applied lines of solution are dried and cured at temperatures of 300.degree. to 400.degree. F. The viscous solution comprises up to about 13 weight percent hydrogen chloride, sufficient organic thickener to provide a viscosity of 2,000 to 3,000 centripoises, and minor amounts, up to about 2 percent, of wetting agent and antifoam agent.
When an acidic solution is used to form the embrittled zones, a problem is encountered. Residual acid remains in the backing after the drying and curing operation even when a volatile acid, such as hydrochloric acid, is used. When label stock having these acid embrittled zones is rolled or stacked for transfer to a label printer, for example, the backing of one layer of label stock comes into contact with the face of another layer of label stock. Residual acid is transferred from embrittled zones of the backing to the face of the adjacent label stock and changes the printing characteristics of the face. When ink is subsequently applied to the label face in a printing operation, the portions of the label affected by the residual acid take on a color or intensity of color different from that of the surrounding regions. The effect is typically seen as a line of lighter or contrasting color running across the face of the printed label. The degree of this effect depends upon the chemical nature of the ink used. Some inks are so sensitive to the acid-caused changes that they cannot be used with the acid-embrittled adhesive products of the prior art.
Another problem encountered in the manufacture of the prior art products is the need to balance carefully the degree of brittleness and the tensile strength of the embrittled zones to provide a balance between good cracking performance upon flexing and avoidance of unwanted cracking.